View source for Classless Inter-Domain Routing

==Background==
===Original Internet Protocol addressing===
All versions of the [[Internet Protocol]] assume that the address is split into a ''locator'' part that tells routers how to move it closer to its destination, and an ''identifier'' that brings it to the specific destination host on the final medium on the route. In the very first version of IPv4,<ref name=RFC760>{{citation
 | title = Internet Protocol
 | id = RFC 760
 | publisher = [[Internet Engineering Task Force]]
 | url = http://www.ietf.org/rfc/rfc760.txt}}</ref>the locator field was a fixed 8 bits, allowing for a maximum of 255 possible locators (i.e., possible internconnected networks0. This fit the early research ARPANET, which principally linked large time-sharing computers; there were no [[personal computer]]s, [[local area network]], or routing within organizations. 

The 8-bit locator very quickly proved inadequate, and a quick fix was applied with RFC 791. By constraining the value of the first three bits of the locator field, without changing the address length, which would need substantial equipment change, it became possible, among other things, to have more locators with different lengths, appropriate for large, medium and small networks:<ref name=RFC791>{{citation
 | title = Internet Protocol
 | id = RFC 791
 | publisher = [[Internet Engineering Task Force]]
 | url = http://www.ietf.org/rfc/rfc791.txt}}</ref>
*Class A: 8-bit locator
*Class B: 16-bit locator
*Class C: 24-bit locator
This change also defined an address space for [[multicasting]].